We came to SoCal from Colorado 4 yrs ago, and from the Bay Area (NorCal) 20 years before that. In all of those places, an I-5 would be "I-5" or just "5" with no "the" in front.
Adapting to life in SoCal's generally been easy as pie, but we have yet to pick up that "the". Guess we still have a...